The present invention relates to a die-casting brass alloy, which is resistant to dezincification.
Dezincification is a problem for brass water fittings, when the water quality varies and maybe is strongly corrosive.
It is known, that it is possible to treat the copper rich alpha-phase in brass against dezincification by means of small additions of arsenic or antimony, whereas the zinc rich beta-phase is not resistant to dezincification.
Thus, it would be logical to keep a high percentage of copper in a brass alloy resistant to dezincification (as an alloy 1 in FIG. 1, showing a portion of the phase diagram Cuxe2x80x94Zn, Hansen, Constitution of binary alloys, New York 1958) in order to minimize or completely avoid the amount of the less corrosive resistant beta-phase. The problem with such an alloy is, that it results in a primary solidification of the alpha-phase in the form of long solidification crystals, so called dendrites, which means, that the beta-phase will form long bands between the alpha-dendrites. This results in two negative consequences:
a) The material will be brittle by heat; and
b) The material will obtain a deep dezincification, since the dezincification will follow the long beta-phase bands.
This phenomenon is thoroughly described in the following scientific article: Arno Louvo, Tapio Rantala, Veijo Tauta, xe2x80x9cThe Effect of Composition on as-cast Microstructure of alfa/beta-Brass and its Control by Microcomputerxe2x80x9d, LISBOA 84, 51 st International Foundry Congress.
FIG. 2, which has been excerpted from this article, describes the problem with brittleness by heat, and FIG. 3, which has been excerpted from the same article, the phenomenon with increasing dezincification depths with an increasing copper content.
In order to avoid the above-mentioned problems the alloy must solidify primarily in the beta-phase as an alloy 2 in FIG. 1, which allows the following advantages:
a) The amounts of micro and macro segregations will be substantially lower for an alloy, which solidifies primarily in the beta-phase. This is caused by the fact, that the diffusion speed in the beta-phase is about 1000 times higher than in the alpha-phase, which is a result of the fact, that its crystal structure has an atom arrangement according to bcc (body-centered-cubic) as compared to the atom arrangement of the alpha-phase fcc (face-centered-cubic).
b) The solidification crystals may be fine grain-treated with boron, which forms fine grains in a very efficient way, and only extremely small amounts of this substance is needed to obtain a fine grain-forming effect. According to experience boron does not have a fine grain-forming effect on brass, which solidifies primarily in the alpha-phase, whereas it is very efficient as far as nucleation of beta-crystals is concerned.
The drawback is, that the beta-phase amount increases in the final casting structure and without a heat treatment it will be difficult to meet the toughest dezincification requirements, which requires a maximal dezincification depth of 100 xcexcm as a separate value. This is true above all for heavy thicknesses of material, shown in FIG. 3.
The information above are known basic facts.
Additional already known techniques are described in WO 89/08725 A1, EP 0 572 959 A1 and MNC manual no. 8, edition 2, September 1987, xe2x80x9cSpecialmxc3xa4ssingxe2x80x9d, page 43.
The object of the present invention is to suggest a way of eliminating the above-mentioned drawbacks.
This object is attained according to the invention by the development of an alloy having the following characteristics.
By balancing copper, zinc, silicon and aluminum in a capable manner it is possible to attain a solidification in the beta-phase and nevertheless avoid the development of continuous beta-phase areas in the finished product. The beta-phase will be found in isolated agglomerates in a matrix of alpha-phase, which is protected against a dezincification due to the arsenic addition. The primary solidification in the beta-phase with the alloy combination according to the invention combined with the high solidification speed of the die-casting limits the size of the agglomerates of the beta-phase in the final casting structure, the agglomerates also in a thick diecasting material with a low solidification speed obtaining an extension, which is clearly less than 100 xcexcm. By means of fine grain-treatment with boron the size of the agglomerates and consequently also the depth of the dezincification can be additionally reduced.